Are you trying to install a new version of SlingPlayer on top of an old one ? If so, can you uninstall the existing app in the normal way ?
Here's a link to the relevant support doc for the full answer:
The short answer is go to Start > Settings > Remove Programs on the device, then trash SlingPlayer Mobile.
If that doesn't work, there may be something else on your device causing a problem and you might need to just wipe it and start over. I know that's a pain, but if your important stuff is backed up to your computer with ActiveSync it's probably the fastest way to fix it if uninstalling normally fails.
Good luck !